Single-photon bound states in atomic ensembles

TL;DR

This paper demonstrates the existence of single-photon bound states in atomic ensembles, linking their presence to transmission zeros and photon correlation divergences, and introduces a dissipative Levinson's theorem for experimental detection.

Contribution

It introduces a method to identify single-photon bound states via transmission phase measurements and formulates a dissipative Levinson's theorem relating bound states to transmission phase winding.

Findings

Bound states correspond to zeros in single-photon transmission.

Divergent bunching occurs at bound state energies.

A dissipative Levinson's theorem relates bound states to transmission phase winding.

Abstract

We illustrate the existence of single-excitation bound states for propagating photons interacting with $N$ two-level atoms. These bound states can be calculated from an effective spin model, and their existence relies on dissipation in the system. The appearance of these bound states is in a one-to-one correspondence with zeros in the single-photon transmission and with divergent bunching in the second-order photon-photon correlation function. We also formulate a dissipative version of Levinson's theorem for this system by looking at the relation between the number of bound states and the winding number of the transmission phases. This theorem allows a direct experimental measurement of the number of bound states using the measured transmission phases.